In a direct contact evaporator for chilling water and/or ice production, water and liquefied refrigerant are brought into direct contact whereby the water is chilled or partially frozen as energy is absorbed by the vaporizing liquefied refrigerant. Direct contact heat transfer allows high rates of energy transfer with small temperature differences by minimizing thermal resistance and maximizing the surface area for energy transfer.
Selection of the refrigerant used in this type of direct contact evaporator is crucial to its performance. Many refrigerants including R-11, R-12, R-22 and R-502 form hydrates when used in direct contact heat transfer with water. A hydrate is a substance in which refrigerant molecules are trapped within the crystal structure of water. Hydrates often form at temperatures above the normal freezing point of water. Typically, approximately 30% by weight of a hydrate is liquid refrigerant. Therefore, the formation of hydrates in a direct contact evaporator represents a consumption of refrigerant which must be replaced for the system to continue operation. Refrigerants are available, however, which do not form hydrates. Refrigerant R-114, C-318 mixtures of R-12 and R-114, n-butane, isobutane and others are all non-hydrate forming. A direct contact evaporator operating with these refrigerants can operate continuously on a fixed charge without requiring a large inventory of refrigerant.
Several problems are associated with the use of conventional lubricated or non-lubricated compression equipment where a direct contact evaporator is used. In these systems, a suction separator is required to minimize the flow of water droplets from the direct contact evaporator into the compressor suction line. In addition, the presence of water vapor in the suction line to the compressor can cause corrosion problems in conventional nonlubricated equipment or it can emulsify the oil in oil lubricated compressors. Also, with oil lubricated equipment an oil recovery system is required on the compressor discharge to minimize the loss of oil to the water or ice in the direct contact evaporator. Finally, non-lubricated compression equipment is typically more expensive and less efficient than lubricated equipment. There is accordingly a need for an improved refrigeration cycle including novel apparatus and methods for producing refrigeration using a direct contact evaporator and a compressor which is lubricated and sealed with an aqueous liquid.